Incandescent light sources are low cost but have low efficiency, and are relatively large requiring large light fittings. Fluorescent lamps in which a gas discharge generates ultraviolet wavelengths which pumps a fluorescent material to produce visible wavelengths, have improved efficiency compared to incandescent sources, but also have a large physical size. Heat generated by inefficiencies in these lamps is typically radiated into the illuminated environment, such that circulating air is used to cool the lamp and provides some heating benefit to the environment.
Light-emitting diodes (LEDs) formed using semiconductor growth onto monolithic wafers can demonstrate significantly higher levels of efficiency compared to incandescent sources. In this specification LED refers to an unpackaged LED die (chip) extracted directly from a monolithic wafer, i.e. a semiconductor element. This is different from packaged LEDs which have been assembled into a package to facilitate subsequent assembly and may further incorporate optical elements such as a hemispherical structure which increases its size but increases light extraction efficiency. To optimise quantum efficiency, extraction efficiency and lifetime, it is desirable to minimise the junction temperature of the LED. This is typically achieved by positioning a heat dissipating structure (or heatsink) on the rear of the LED to achieve extraction of heat from the chip into an ambient environment. Heat is not typically extracted in the same direction as the light output direction. For recessed devices, the heat dissipating structure does not benefit from natural air flow present in the illuminated environment, reducing its extraction efficiency and increasing cost. Further, the heat may be used to heat walls and/or ceilings rather than the air in the illuminated environment.
In lighting applications, the light from the emitter is directed using a luminaire structure to achieve the light output directional distribution. The angular variation of intensity is termed the directional distribution which in turn produces a light radiation pattern on surfaces in the illuminated environment and is defined by the particular application. Lambertian emitters achieve light to the flood a room. Non-Lambertian, directional light sources use a relatively small source size lamp such as a tungsten halogen type in a reflector and/or reflective tube luminaire, in order to achieve a more directed source. Such lamps efficiently use the light by directing it to areas of importance. These lamps also produce higher levels of visual sparkle, in which the small source provides specular reflection artefacts, giving a more attractive illumination environment. Further, such lights have low glare, in which the off-axis intensity is substantially lower than the on-axis intensity so that the lamp does not appear uncomfortably bright when viewed from most positions.
Directional LED elements can use reflective optics (including total internal reflective optics) or more typically catadioptric (or tulip) optic type reflectors, as described for example in U.S. Pat. No. 6,547,423. Catadioptric elements employ both refraction and reflection, which may be total internal reflection or reflection from metallised surfaces. A known catadioptric optic system is capable of producing a 6 degree cone half angle (to 50% peak intensity) from a 1×1 mm light emitting element, with an optical element with 13 mm final output diameter. The increase in source size arises from conservation of brightness (étendue) reasons. Further, such an optical element will have a thickness of approximately 11 mm, providing a bulky illumination apparatus. Increasing the cone angle will reduce the final device area and thickness, but also produces a less directional source.